1. Field of the Invention
The present invention relates to diamonds and diamond-like materials and to products made thereof. In another aspect, the present invention relates to planarized diamond films and diamond-like material films and to products made thereof. In still another aspect, the present invention relates to polished and planarized diamond films and diamond-like material films and to products made thereof. In even another aspect, the present invention relates to polished and planarized polycrystalline diamond films, to products made thereof, and to a method of polishing and planarizing polycrystalline diamond films.
2. Description of the Related Art
Diamond films are finding use in a multitude of applications, including electrical applications. To be useful, such films must be not only highly polished but must also have a high degree of planarity.
The degree of polish generally refers to the average surface roughness, whereas planarity generally refers to the waviness of the film or the variation in thickness of the film.
While techniques do exist for the polishing of diamond films, they generally do not provide the required degree of planarity.
The applicability of diamond films are a result of their physical and chemical properties which render them suitable for use in a wide range of applications. For example, natural diamonds are the hardest substance known and exhibit low friction and wear properties. Specifically, a natural diamond's thermal conductivity, thermal diffusivity properties, electrical resistivity and microhardness invite its application in various applications.
As the physical and chemical properties of synthetic diamond films have been found to be comparable to those of bulk natural diamond, it is believed that diamond films will also be used in a broad range of applications.
For example, it has been reported that typical chemical vapor deposited films have electrical resistivities greater than 10.sup.13 .OMEGA.-cm, a microhardness of about 10,000 HV or higher, a thermal conductivity of about 1100 W m.sup.-1 K.sup.-1 or higher, and thermal diffusivity of 200 to 300 mm.sup.2 /s. These reported values for deposited diamond films compare favorably to the same properties of natural diamond, i.e, resistivities in the range of 10.sup.7 to 10.sup.20 .OMEGA.-cm, a microhardness in the range of 8,000 to 10,400 HV, a thermal conductivity in the range of 900 to 2000 W m.sup.-1 K.sup.-1, and a thermal diffusivity of 490 to 1150 mm.sup.2 /s. Thermal gravimetric analysis demonstrates that the oxidation rates of diamond films in air are lower than those of natural diamond. Additionally, it is reported that the minimum starting temperature for the oxidation of microwave-assisted chemical vapor deposited diamond films is about 800.degree. C., as evidenced by weight loss, while the morphology shows visible oxidation etching pits at temperatures as low as 600.degree. C.
Diamond films are not naturally occurring, but rather must be manufactured using any of a host of techniques, such as, chemical vapor deposition, physical vapor deposition, plasma spray or cathode sputtering.
The growth of diamond film on a nondiamond substrate is initiated by nucleation either at randomly seeded sites or at thermally favored sites. Based on growth temperature and pressure conditions, favored crystal orientations dominate the competitive growth process. As a result, the grown films are polycrystalline in nature with relatively large grain size, generally greater than one micron, terminating in rough surfaces with the roughness ranging from about a few tenths of a micron to tens of microns. Such films offer insufficient planar areas and will likely not be suitable for most applications, in particular for thermal management applications.
While acceptable surface smoothness is perhaps the major constraint for the widespread use of diamond films in thermal management, electrical, optical and tribological applications. Another very important characteristic of polished diamond surfaces is waviness, the periodic or aperiodic wave-like variation from a perfectly planar surface, which is generally much larger and wider than the roughness. Depending upon the application of the product, waviness may be undesirable while minute scratches can be tolerated. For example, in gauge blocks, the polished steel surface has little waviness, but on a microscopic scale is scratched.
As is well known, diamond is the hardest substance known and is therefore difficult to polish. In general, abrasive polishing techniques require the use of a mating/polishing grit material of equal or greater hardness than the material to be polished. While other materials may be polished utilizing harder substances, diamond films are generally abrasively polished only with diamonds.
Various prior art methods have been suggested to improve upon the abrasive polishing of diamond films. For example, in "Polishing diamonds in the presence of oxidizing agents", Thornton et al., Diamond Research 1974, Supplement to Industrial Review, pp. 39 (1974), it is disclosed that the polishing of a natural diamond stone with diamond powder and an iron scaife may be enhanced by first applying a concentrated aqueous solution of potassium nitrate on the iron scaife.
Typically, polishing with diamond powder commences with a relatively coarse hard powder which continuously scratches the surface of the material being polished until all of the scratches remaining on the surface are as small as can be made with that size powder. The next step is to polish with a smaller size powder until all of the larger scratches are removed and the only remaining scratches are the smallest that can be produced with this second size powder. This continues with successively smaller powder sizes until the desired degree of polishing is obtained. Obviously, the degree of polish of the finish will always depend on the size of powder utilized.
Hall et al. U.S. Pat. No. 4,662,348, issued May 5, 1987 discloses a method for burnishing a diamond which eliminates the necessity of diamond powder. As disclosed, a polished diamond surface is obtained by rubbing the surface of the diamond to be polished against a smooth complementary diamond surface with sufficient pressure and velocity to heat the surface being polished above the spontaneous thermal degradation temperature of the diamond.
Unfortunately, traditional abrasive polishing methods utilizing diamond powder or complementary surfaces are unsuitable for diamond films because of extremely low polishing rates and preferential polishing along specific crystal directions leaving grooves on the surface.
As alternatives to the traditional abrasive polishing methods, various physical and chemical means have been explored to etch or polish diamond films. These alternative methods can be generally classified as thermochemical, chemomechanical or plasma/ion beam/laser polishing.
Thermochemical techniques generally involve mechanical contact of the diamond film to certain metals at elevated temperatures. In this case, the diamond surface is put in, not only mechanical, but also thermal contact, typically with a spinning hot plate. Commonly, iron is the preferred plate material since, above 723.degree. C., the solubility of carbon in an iron matrix increases, and thus, unwanted diamond asperities can be dissolved in the iron matrix. However, the technique offers polished films with a non-diamond surface having inter-grain contamination.
Plasma, ion beam, and laser polishing are non-contact polishing techniques, which generally do not require bulk sample heating and can be used on nonplanar surfaces. To date, the material removal rates of these techniques have been small. Additionally, these techniques require a controlled environment, generally a vacuum, and require expensive equipment.
It is well known that a diamond can be etched by exposure to an etching agent such as potassium nitrate or potassium chlorate at elevated temperatures, generally above 600.degree. C. However, etching generally results in a deeply pitted diamond surface with the etching occurring preferentially at dislocations and other defects.
For example, Purohit et al. U.S. Pat. No. 5,133,792, issued Jul. 28, 1992, discloses a method of cleaning and refining by soaking diamonds in caustic or acidic solutions for durations of possibly more than a day at temperatures in the range from about 200.degree. C. to about 500.degree. C.
Chemical mechanical methods generally include a first polishing step in which the diamond film is coarsely polished by lapping against a polycrystalline alumina plate in the presence of fused potassium nitrate. Next, the diamond film is finely polished by lapping against another diamond film in the presence of fused potassium nitrate. However, the resultant diamond film has amorphous non-diamond contamination on the surface, probably from the mating diamond film surface or from execessive heating.
While various prior art methods and apparatus for polishing diamond films and products from diamond films exist, they each suffer from one or more disadvantages. The ideal processing method would both polish and planarize, as well as be non-contaminating to the diamond surface. Additionally, whether or not the techniques contaminate the diamond surface is important.
For example, thermochemical, ion beam, and mechanical lapping methods all achieve reasonable levels of polishing but fail to planarize the diamond film. Additionally, while laser methods produce polishing on the order of 0.05 microns, contamination by the formation of graphitic or diamond-like carbon layers occurs. While ion beam methods produce a surface finish on the order of 0.005 microns, the surface roughness is non-uniform due to ion-beam non-uniformity. Plasma methods achieve highly non-uniform polishing and contamination in the form of residue formation on the surface in grain boundaries. Mechanical lapping methods produce polishing on the order of 0.02 microns, but cause surface structural deformations, a type of defect, on micro scale.
Also, while the thermochemical technique offers a fine surface finish, surface non-uniformities are introduced from the abraiding metal surface. Contamination occurs from the formation of a diamond-like carbon layer and metal residue in the grain boundaries.
Although not related to the diamond film processing art area, a method of creating a planarized surface on top of a silicon wafer having electrical components is disclosed in "Searching for Perfect Planarity", Peter H. Singer, Semiconductor International, March 1992. However, the disclosed method does not create a planarized surface from the silicon wafer itself, but rather creates a planarized polymer layer on top of the silicon wafer, which layer encapsulated the electrical components supported by the wafer.
Thus, while the above described methods of processing diamond films offer polished diamond surfaces and provide some improvement in the waviness or planarization, they still suffer from one or more deficiencies.
Additionally, while some techniques can provide improvement in local planarization, i.e., across local portions of the substrate, most of them cannot provide truly global planarization, i.e. across the entire substrate.
Also, most polishing techniques generally result in the creation of imperfections on the diamond film surface, or in the exposure of subsurface microcavities between diamond crystals which also results in imperfections on the diamond film surface. This fact puts a natural limit on the achievable degree of surface finish by polishing.
Therefore, there is a need in the art for an improved method of processing diamonds and diamond-like materials.
There is another need for improved products made from diamond films and diamond-like material films.
There is yet another need in the art for improved planarized diamond films and diamond-like material films, improved products made thereof, and for an improved method of planarizing diamond films and diamond-like material film products.
There is even another need in the art for improved polished diamond films and diamond-like material films, improved products made thereof, and an improved method of polishing diamond films and diamond-like material film products.
These and other needs in the art will become readily apparent to one of skill in the art of this invention upon reading this specification.